As shown in FIG. 1, in the exposure process of fabricating RGB film of a color filter of liquid crystal display panel, four-point alignment method is often used to align a mask 6 with a glass substrate 7, that is, the mask 6 is aligned with the glass substrate 7 by aligning alignment marks at four corners of the mask 6 with alignment marks at four corners of the glass substrate 7, and an exposure is occurred only if the four points are simultaneously aligned.
The alignment of above points is achieved by a detection system. The detection system includes a detection lens 1, a semi-transparent reflector 2 and a light source 3; light emitted by the light source 3 passes through the semi-transparent reflector 2 and a negative pressure mask 5 and vertically irradiates on the mask 6 and the glass substrate 7; the detection lens 1 collects images of a mask alignment mark 8 on the mask 6 and a substrate alignment mark 9 on the glass substrate 7 so as to form a mask alignment image 10 and a substrate alignment image 11, and detects an alignment degree of the substrate alignment mark 9 and the mask alignment mark 8 by detecting a matching degree of the mask alignment image 10 and the substrate alignment image 11.
During aligning the above mask 6 with the glass substrate 7, if there are four substrate alignment images 11 having inconsistent gray scales in the detection system in one alignment process, the detection system would judge that the mask 6 is not aligned with the glass substrate 7, then it needs a manual adjustment, which allows the detection system having a low degree of automatization, thereby productivity is affected. In prior art, regions of four substrate alignment marks 9 on the glass substrate 7 are usually coated with photoresist so that their gray scales are consistent. However, for reason of cost saving, the coated regions are reduced in actual application, thus it cannot ensure that the whole regions of the substrate alignment marks 9 are coated. Therefore, the detection system has a high misalignment rate and a low degree of automatization.